1. Field of the Invention
The present invention relates to video processing technology. In one aspect, the present invention relates to decompression of digital video information.
2. Description of the Related Art
Because video information requires a large amount of storage space, video information is generally compressed. Accordingly, to display compressed video information which is stored, for example on a CD-ROM or DVD, the compressed video information must be decompressed to provide decompressed video information. The decompressed video information is then provided in a bit stream to a display. The decompressed bit stream of video information is typically stored as a bit map in memory locations corresponding to pixel locations on a display. The video information required to present a single screen of information on a display is called a frame. A goal of many video systems is to quickly and efficiently decode compressed video information so as to provide motion video by displaying a sequence of frames.
Standardization of recording media, devices and various aspects of data handling, such as video compression, is highly desirable for continued growth of this technology and its applications. A number of (de)compression standards have been developed or are under development for compressing and decompressing video information, such as the Moving Pictures Expert Group (MPEG) standards for video encoding and decoding (e.g., MPEG-1, MPEG-2, MPEG-3, MPEG-4, MPEG-7, MPEG-21) or the Windows Media Video compression standards (e.g., WMV9). Each of the MPEG and WMV standards are hereby incorporated by reference in its entirety as if fully set forth herein.
In general, video compression techniques include intraframe compression and interframe compression which operate to compress video information by reducing both spatial and temporal redundancy that is present in video frames. Intraframe compression techniques use only information contained within the frame to compress the frame, which is called an I-frame. Interframe compression techniques compress frames with reference to preceding and/or following frames, and are typically called predicted frames, P-frames, or B-frames. Intraframe and interframe compression techniques usually use a spatial or block-based encoding whereby a video frame is split into blocks for encoding (also referred to as a block transformation process). For example, an I-frame is split into 8×8 blocks. The blocks are coded using a discrete cosine transform (DCT) coding scheme which encodes coefficients as an amplitude of a specific cosine basis function, or some other transform (e.g., integer transform). The transformed coefficients are then quantized, which produces coefficients with non-zero amplitude levels and runs (or subsequences) of zero amplitude level coefficients. The quantized coefficients are then run-level encoded (or run length encoded) to condense the long runs of zero coefficients. The results are then entropy coded in a variable length coder (VLC) which uses a statistical coding technique that assigns codewords to values to be encoded, or using some other entropy encoding techniques, such as a Context-based Adaptive Binary Arithmetic Coding (CABAC), Context Adaptive Variable Length Coding (CAVLC) and the like. Values having a high frequency of occurrence are assigned short codewords, and those having infrequent occurrence are assigned long codewords. On the average, the more frequent shorter codewords dominate so that the code string is shorter than the original data. Thus, spatial or block-based encoding techniques compress the digital information associated with a single frame. To compress the digital information associated with a sequence of frames, video compression techniques use the P-frames and/or B-frames to exploit the fact that there is temporal correlation between successive frames. Interframe compression techniques will identify the difference between different frames and then spatially encode the difference information using DCT, quantization, run length and entropy encoding techniques, though different implementations can use different block configurations. For example, a P-frame is split into 16×16 macroblocks (e.g., with four 8×8 luminance blocks and two 8×8 chrominance blocks) and the macroblocks are compressed. Regardless of whether intraframe or interframe compression techniques are used, the use of spatial or block-based encoding techniques to encode the video data means that the compressed video data has been variable length encoded and otherwise compressed using the block-based compression techniques described above.
At the receiver or playback device, the compression steps are reversed to decode the video data that has been processed with block transformations. FIG. 1 depicts a conventional system 30 for decompressing video information which includes an input stream decoding portion 35, motion decoder 38, adder 39, frame buffer 40, and display 41. Input stream decoder 35 receives a stream of compressed video information at the input buffer 31, performs variable length decoding at the VLC decoder 32, reverses the zig-zag and quantization at the inverse quantizer 33, reverses the DCT transformation at IDCT 34 and provides blocks of staticly decompressed video information to adder 39. In the motion decoding portion 38, the motion compensation unit 37 receives motion information from the VLC decoder 32 and a copy of the previous picture data (which is stored in the previous picture store buffer 36), and provides motion-compensated pixels to adder 39. Adder 39 receives the staticly decompressed video information and the motion-compensated pixels and provides decompressed pixels to frame buffer 40, which then provides the information to display 41.
With conventional video encoder and decoder designs, blocking artifacts (noticeable discontinuities between blocks) can be introduced into a frame from the block-based transform, motion compensation, quantization and/or other lossy processing steps. Prior attempts to reduce blocking artifacts have used overlap smoothing or deblocking filtering (either in-loop or post processing) to process frames by smoothing the boundaries between blocks. For example, with the WMV9 standard, it is specified that overlap smoothing and in-loop deblocking are processed on the whole picture to reduce blocking artifacts. With WMV9 decoding enabled, overlap smoothing is done only on the 8×8 block boundaries, starting with smoothing in the vertical direction for the whole frame, and then overlap smoothing is performed in the horizontal direction for the whole frame. Next, in-loop deblocking, when enabled, is done in this order: (i) all the 8×8 block horizontal boundary lines in the frame are filtered starting from the top line; (ii) all 8×4 sub-block horizontal boundary lines in the frame are filtered starting from the top line; (iii) all 8×8 block vertical boundary lines are filtered starting with the leftmost line; and (iv) all 4×8 sub-block vertical boundary lines are filtered starting with the leftmost line. Prior approaches use two passes on the entire frame, where the first pass is to perform overlap smoothing, and the second step is for in-loop deblocking. While there may be other requirements (e.g., involving a parameter PQUANT and block types) that also apply when determining whether or not to do the processing of the individual step, the goal of these processes is to smooth over the edges of 16×16 macroblock, 8×8 blocks or 4×4 sub-blocks, thereby removing the artifacts of blockiness introduced by the 2D transform and quantization.
With processor-based approaches for handling video decompression, the addition of a smoothing or deblocking function is a computationally intensive filtering process. This order of processing can be done in software when there is a large memory buffer to hold a frame (e.g. VGA size of 640×480 pixels, equivalent to 307 kbytes). On the other hand, hardware-based approaches for decoding have not performed smoothing and deblocking at the same time, and have performed deblocking on the frame as a whole, which requires a large local memory, imposes significant bus bandwidth requirements and sacrifices memory access time. Consequently, a significant need exists for reducing the processing requirements associated with decompression methods and for improving the decompression operations, including specifically the overlap smoothing and/or deblocking filter operations. Further limitations and disadvantages of conventional systems will become apparent to one of skill in the art after reviewing the remainder of the present application with reference to the drawings and detailed description which follow.